As is well known, a periodic signal has a frequency, a magnitude and a phase. When analyzing periodic signals, the amplitude and phase of the signal is, for mathematical convenience, represented by a phase vector, called a phasor, that rotates about an origin. The angle of the phasor with respect to a reference indicates the phase of a signal at a particular time, and its length or magnitude indicates the amplitude of the signal. In view of this phasor representation, periodic signals are often spoken of in the art as "vectors," and the process of phase shifting periodic signals is commonly referred to as "vector modulation". In the following description, as in the art, the term "vector" refers both to actual periodic signals as well as to an abstract mathematical representation of periodic signals.
Vector analysis of periodic signals demonstrates several concepts important to understanding the present invention. When, for example, two or more signals are summed, the amplitude and phase of the resulting signal may be determined by adding the vectors of the phasors of each signal. This vector representation of summed signals thus suggests a common method of vector modulation or phase shifting referred to as a variable amplitude phasor approach. In this approach, an output signal having a given phase shift may be generated by properly splitting and shifting the input signal and summing the two resulting split signals. For example, equally splitting an input signal into two identical signals, shifting one by 90.degree. and the other by 180.degree., and adding the two delayed signals produces the input signal phase shifted by 135.degree.. In this example, by varying the amplitude of one of the signals, thereby varying the length (magnitude) of its vector representation, the input signal can be phase shifted through all the angles between 90.degree. and 180.degree..
Although phase shifters and vector modulators have a wide range of uses, one important use is in steering a beam of phased array antenna. A phased array antenna is comprised of a plurality of antenna elements, each of which receives signals identical in almost every respect except for their phases. A phased array antenna generates a beam in a given direction by constructively combining radiation emitted from each individual antenna element. To steer the beam, the phases of the signals applied to each element are appropriately shifted by phase shifters. Accurately pointing the beam and, at the same time, suppressing radiation sidelobes depends on accurately controlling the phase shifts between antenna elements.
In a paper entitled "A Monolithic 10-GHz Vector Modulator", published by the IEEE in 1983 as part of its GaAs IC Symposium, Eric W. Strid describes a phase controller (hereinafter referred to as the "Strid vector modulator") that is monolithically implemented as a single integrated circuit utilizing the variable amplitude phasor approach. This single integrated circuit is small and light, and therefore has the advantage of being usable with phased array antennas on moving platforms such as military aircraft and spacecraft.
The circuit of the Strid vector modulator includes a first and a second tapped lumped delay line. The delay lines are like those used by a transversal filter for matched filter detection of PCM signals. The periodic signal to be selectively phase shifted is applied to an input of one of the delay lines. Each delay line is comprised of a number of series connected signal delayers to generate phase shifts. The amount of delay between each signal delayer in the delay lines is chosen such that the phase of a periodic signal at a center frequency in the band of interest is delayed by 90.degree. or 120.degree. for each delayer as the signal propagates through each delay line. A plurality of field effect transistors (FETs), connect each delayer of the first delay line to a corresponding delayer on the second delay line.
The paths through the delay elements and the FETs form a number of vector channels. Each vector channel provides to the outputs of the Strid vector modulator variably phase shifted versions of the input signal for summing. The signal generated by each vector channel differs in phase by a predetermined amount, usually in increments of 90.degree. or 120.degree.. By turning the FETs "on" and "off" and adjusting the amplitude of the applied voltage to each FET, different vectors of differing magnitudes and phases may be created and added to vary and control the phase shift of the input signal. Therefore, the Strid vector modulator functions as an analog phase shifter capable of producing any angle between 0.degree. and 360.degree..
The Strid vector modulator satisfies many requirements for phase shifters used for phased array antennas aboard moving platforms, namely that the vector modulators be small and light, and consume little power. However, the vector modulation bandwidth of the Strid vector modulator is severely narrow, especially for signals in the microwave range, as a result of the utilization of signal delayers for creating phase shifts.
At a given center frequency the vectors of the periodic signal in each of the four vector channels are separated by the required 90.degree. of phase shift. However, use of frequency dependant signal delayers to generate phase shifts results in different transit times through the vector channels for different frequencies of the input signal. Thus, deviations in the input frequency causes skewing of the frequency versus phase response of the Strid vector modulator resulting in phase distortion in the output signal. As the frequency of the input periodic signal applied to a Strid vector phase modulator deviates from the center frequency, the angles between the vectors of the signal samples begin to deviate and drift substantially and are no longer fixed at 90.degree. apart.
When used in conjunction with phased array antennas, the Strid vector modulator introduces intolerable phase errors and distortions in the signal that seriously effect array beam pointing accuracy and the suppression of array side lobe radiation. The Strid phase shifter is, therefore, unsuitable for use in highly accurate phased array antennas transmitting broadband periodic signals.
It is theoretically possible to correct the Strid phase errors and distortions by varying the applied FET voltage as the frequency of the vector modulator input signal changes. However, aside from the impracticality of dynamically adjusting the amplification of the FETs as a function of input signal frequency variation, the extra circuitry and complexity required to provide adjustable FET amplification to correct the resulting phase distortions of the Strid device leads away from the goals of a small, simple, low power device capable of monolithic implementation to achieve reproducible performance and mass production.
Accordingly, there is a need for a small, simple, low power vector modulator having consistent broadband frequency response further capable of being implemented on a monolithic chip.